When a curved surface of a three-dimensional structure such as an aluminum ball tank is automatically welded, welding grooves are formed on the front and rear faces of the object to be welded, and the frontal and rear grooves are welded up.
However, when the rear face is welded, the rear grooves are placed in an unfavorable form, and therefore, the welding bead is worsened, thereby deteriorating the mechanical strength of the welded portion.
Conventionally in the case of a curved portion such a ball tank, it is impossible to automatically access a welding torch to the object to be welded, and therefore, separate devices have to be used for the respective welding postures, while the welding with an upward posture has to be manually welded, thereby making it liable to cause accidents and other inconvenience.
The conventional wire spool for the automatic welding apparatus is exposed to the open air, and therefore, the welding wire is oxidized, or contaminated with foreign materials. Therefore, if the welding wire is left under the open air for a long time, blow holes are formed, with the result that welding defects are produced. Further, during winter seasons and raining seasons, and during seasons showing severe temperature differences, a dew formation is caused to aggravate the rate of welding defects.